Add 'isCodeGenOnly' bit to Instruction .td records.

Add 'isCodeGenOnly' bit to Instruction .td records.
 - Used to mark fake instructions which don't correspond to an actual machine
   instruction (or are duplicates of a real instruction). This is to be used for
   "special cases" in the .td files, which should be ignored by things like the
   assembler and disassembler. We still need a good solution to handle pervasive
   duplication, like with the Int_ instructions.

 - Set the bit on fake "mov 0" style instructions, which allows turning an
   assembler matcher warning into a hard error.

 - -2 FIXMEs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78731 91177308-0d34-0410-b5e6-96231b3b80d8